Create a series of three clinics for “youth in dressage” , and make it completely grass roots: reach out to 4-H and pony club. Charge next to nothing, offer scholarships if kids can write a little essay that explains why they are interested in dressage. Pay instructors for a whole day of clinic, possibly two. Make sure to include a lecture. Make sure the instructors are actually kid friendly, and make things fun.
Emphasize strongly that this is not stuff for someone who already rides in the sport, it is inclusive of anyone who has any interest whatsoever. They can bring whatever tack they’ve already got.
That’s a start
Then you need a way to keep the kids going/interested. Again, create a scholarship situation. There might be some instructors who would donate a portion of their fee and the local dressage club can pay the rest. Or even the local dressage club pays half of a series lot of lessons for interested kids.
See if you can get a team of kids to work as “helpers” at shows. They get coupons for Lessons.
Sponsor youth only classes at local schooling shows. LOWER THE FEES. Get a kids-only clinic going after the show.